For some time, known discharge valve apparatuses attached to the discharge opening of a flush water tank for storing flush water for flushing a toilet have included those in which, as described, for example, in JP 2002-70109A (Patent Document 1), two locations on a discharge valve body portion are affixed using affixing hardware such as bolts to the discharge opening of a flush water tank on what is known as a one-piece flush toilet, in which the flush water tank portion and the toilet main body portion are integrally formed.
It is generally necessary in one-piece flush toilets to assure the greatest possible instantaneous flow rate of flush water supplied to the toilet main body portion from the flush water tank portion, but in the above-described conventional discharge valve apparatus the structure provides affixing hardware inside the discharge flow path of the discharge valve body portion, so the discharge flow path is narrowed by the area occupied by the affixing hardware. Therefore various design measures are employed in the toilet main body portion or the flush water tank portion to assure high instantaneous flow rate, such as setting a high initial water level in the flush water tank prior to start of discharge to create a high flush water head pressure, or setting a large diameter for the discharge opening.
On the other hand, known conventional flush toilets have included, in addition to the above-described one-piece flush toilets, what is known as two-piece flush toilets, wherein the flush water tank and toilet body are separately formed in advance, then the two are assembled.
In such two-piece flush toilets, unlike one-piece flush toilets, the discharge valve body portion can be affixed from the outside in advance to the discharge opening of a flush water tank with affixing members, prior to assembling the flush water tank to the toilet body, therefore no affixing hardware need be disposed inside the discharge flow path, and an appropriate flush water tank can be selected and assembled according to toilet body specifications, usage conditions, and required flush capability, so that the diameter of the flush water tank discharge opening can be standardized to predetermined measurements, and a common discharge valve apparatus can as much as possible be adopted to handle a diversity of toilet designs.